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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Gordon
Yes, that should be fine... There's already an example using a Web Bluetooth connection at http://www.espruino.com/Bangle.js+Data+Streaming#bonus-2-heart-rate-monitoring-graph
But for reliability, using Advertising would be easier as you say. There's already some code to handle button presses to a Pi at http://www.espruino.com/BLE+Advertising so tweaking that to do HRM should be fine.
Heart rate monitor uses about 2.5mA so I'd hope you should be able to do it non-stop for maybe 3 days or so